Hard rock drilling of deep wells is typically carried out using a rotary drill string. The rotation speed of a drill string is limited to around 150 rpm, and this technique requires a heavy drill pipe to carry the load and to apply high thrust to the rock. Drill motors operate at a relatively higher rotary speed, and provide faster drilling rates as compared to rotary drilling Positive displacement motors are used for geothermal drilling, but these tools incorporate elastomeric stators that degrade at high temperatures.
Mud turbines (turbodrills) offer higher rotary speeds, up to 1000 rpm, which improves the rate of penetration (ROP) of bits with small cutter exposure. These mud turbines are also capable of operating under high temperature conditions, with a greater service life than the positive displacement motors employing elastomeric stators. Recent developments in bearings have dramatically improved motor life in mud turbines, allowing runs of hundreds of hours. Mud turbine motors have a limited pressure capacity, because the internal pressure must be supported by the motor bearings. This pressure leads to high bearing loads, and wear when the bit is off-bottom.
It would be desirable to provide an improved drill motor capable of operating at an ultra-high rotary speed (i.e., well over 1000 rpm).